Electromagnetic structures for hightemperature service



Jan. 23, 1968 s u s ET AL 3,365,688

ELECTROMAGNETIC STRUCTURES FOR HIGH-TEMPERATURE SERVICE Original FiledFeb. 14, 1962 NICKEL OR STAINLESS FUSED GLASS STEEL CLADDING UNFIREDENAMEL SILICONE AND POWERED GLASS COPPER CORE COPPER CORE UNFIRED: STATEFIRED STATE TWO GLASS FIBER SERVINGS FIBERS AND FUSED GLASS CLADDINGosmous OF UNFIRED BOND= SILICONE I 'AND POWDERED I GLASS COPPER COPPERCORE CORE UNFIRED STATE FIRED STATE INVENTORS HARRY L. SAUMS WESLEY W.PENDLETON BY J g/ r M M ATTORNEYS 3,365,688 ELECTROMAGNETIC STRUTURESFOR HIGH- TEMPERATURE SERVICE Harry L. Saums, North Muskegon, and WesleyW. Pendleton, ll'luskegon, Mich, assignors, by mesne assignments, toAnaconda Wire and Cable Company, a corporation of Delaware Originalapplication Feb. 14, 1962, Ser. No. 173,115, now Patent No. 3,273,225,dated Sept. 20, 1.966. Divided and this application Feb. 15, 1965, Ser.No. 547,105

6 Qlaims. (Cl. 336-222) This is a divisional application of theapplication Ser. No. 173,115 filed Feb. 14, 1962, now Patent 3,273,225,which in turn is a continuation-in-part application of a copendingapplication Ser. No. 763,187 filed Sept. 25, 1958, now abandoned.

This invention relates to electromagnetic structures and, moreparticularly, to the manufacture of insulated electromagnetic windingsin which the insulation is an inorganic dielectric. The inventionprovides improved insulated electromagnetic windings which are capableof being continuously operated at temperatures of from 300 C. to 500 C.,or more, and which may be successfully empolyed in electric generators,motors and other electromagnetic apparatus designed for service at hightemperatures.

One of the most important of the limiting factors which must beconsidered in the design of any electromagnetic machine or apparatus isthe temperature at which the insulation surrounding the magnet wire ofthe electromagnetic winding either is thermally deformed or undergoesthermal degradation. Gperation of the machine at or near thistemperature sets in motion a chain of events which ultimately results incomplete failure of the machine, for when the insulation of theelectromagnetic winding melts or is thermally degraded, the adjacentturns of magnet wire which make up the winding come into contact witheach other and with the frame of the machine, thereby creating shortcircuits which markedly decrease the impedance of the winding. Thisdecrease in impedance is accompanied, not only by a decrease in therequired magnetic field, but also by an increase in power consumptionwhich, in turn, results in a further increase in the temperature of thesystem and ultimately in a complete failure of the electromagnet andhence of the machine. Consequently, the design of electromagneticmachinery for high-temperature service requires specially insulatedwindings, the insulation of which is not subject to either plastic flowor thermal degradation at the temprature at which the machine isdesigned to operate.

Where the selection of the proper dielectric for insulating theelectromagnetic winding the only problem which is faced in themanufacture of electromagnetic machines for high-temperature service,then electric motors for example, capable of operating at very hightemperatures could be designed and fabricated, since a number ofdielectrics, notably the inorganic dielectrics, possess both therequisitt thermal stability and insulating properties necessary forthese windings. However, other important limiting factors than thethermal characteristics of the magnet wire insulation influence electricmotor design, the most important of which is maintaining the size andweight of the motor at a minimum for a given power output. As a generalrule, the trend in electric motor design is toward smaller, lighterelectric motors having increased power capacity.

Small, lightweight electric motors require correspondingly small andcompact electromagnetic windings. However, most of the organic polymersused to insulate magnet Wire, from which electromagnetic windings areformed, suffer serious deterioration at temperatures not 3,355,63latentecl Jan. 23, 1868 far above 200 C. Although other organic polymerdielectries, notably the polytetrafluoroethylenes, are thermally stableat much higher temperatures, it is very difiicult, if not impossible, toapply thin films of these polymers to magnet wire. Because almost all ofthe small and compact electromagnetic windings presently manufacturedfor high temperature service are insulated with organic polymerdielectrics which melt or undergo deterioration at temperatures at ornear 200 C., the maximum temperature at which electric motorsspecifically designed for high temperature service can be safelyoperated is approximately C., or lower.

In theory, this maximum temperature could be increased by using certaininorganic dielectrics to insulate the magnet wire since as a generalrule inorganic insulation is far more stable at high temperatures thanis organic insulation of the same dielectric value. However, magnet wirewhich is insulated with an inorganic dielectric, such as mica,porcelain, glass, asbestos, or quartz, cannot be satisfactorily formedinto an electromagnetic winding since the inorganic insulation lackssufiicient mechanical strength to withstand the hazards of winding. Forexample, when magnet wire insulated with a serving of braided glassfibers is formed into a winding, the individual glass fibers, beingbrittle and smooth, crack and invariably separate from one another,exposing the underlying magnet wire. In fact, merely winding the magnetwire results in cracking of the insulation, the more fragile or brittlethe insulation the more pronounced its breakage. Since brittlencss ischaracteristic of all inorganic dielectrics, the use of inorganicinsulating materials as the sole insulation for electromagnetic windingshas been virtually restricted to only the very largest electromagnetsdevoid of small radii.

The present invention provides an improved method for manufacturinginsulated electromagnetic windings which are capable of continuousoperation at temperatures up to 500 C. or more, and in which the soleinsulation is an inorganic insulating material. In accordance with theinvention, these electromagntic windings are manufactured by applying aflexible abrasion-resistant insulation to magnet wire, the insulationcomprising a covering of fibrous inorganic dielectric material initiallybonded in place by an organic varnish or enamel, and then forming thisinsulated wire into a winding. No matter how tightly the wire is wound,the organic varnish or enamel reinforces the inorganic fibers andcompletely prevents breakage or loosening of the fibrous inorganicdielectric material and exposure of the underlying conductor. After thewinding has been formed, it is heated to a temperature above thepyrolysis point of the organic varnish for a sufficient period of timeto pyrolytically volatilize all organic matter from the covering andleave the formed winding insulated solely by the inorganic dielectricmaterial. Because the final insulation is solely inorganic and becauseit is thermally stable at temperatures as high as 500 or even more,electromagnetic windings made in accordance with the invention may nowbe successfully employed in any electromagnetic apparatus or machinedesigned to operate at or below such temperatures.

It has particularly been found that fibrous inorganic material which isserved or in any way applied to the conductor can be bonded thereon in anovel manner by coating it with a slurry of organic varnish pigmentedwith at least one finely-divided glass. When such a bonding material isused, the winding subsequently formed with these conductors is firstheated to a temperature above the pyrolysis point of the organic varnishbut below the fusion point of the finely-divided glass to pyrolyticallyvolatilize all organic matter, and is then heated to a temperature abovethe fusion point of one of the finelydivided glasses but below thefusion point of the fibrous inorganic material, thereby fusing the glassabout the fibrous inorganic material. Alternatively, it has been foundthat by coating a conductor with an enamel consisting of an organicvarnish pigmented with finely-divided glass fibers of a particular size,a mat structure is produced by the glass fibers such that they tend toadhere to the conductor between the pyrolysis step and the fusing step,and do not fall off as would be the case if ordinary powder or flakeforms of glass were employed. When the finely-divided glass particlesare used there is little or no fall-out of glass particles uponapplication of the glass slurry as was often the case when denseparticles were used.

The magnet wire employed in manufacturing electromagnetic windings inaccordance with the invention may be fabricated from any metallicconductor, such as copper, aluminum or bronze, which is sufiicientlyflexible to be formed into a winding. In some instances it isadvantageous to employ a metallic conductor which is sheathed orjacketed with a thin layer of a dissimilar metal or with a metallicoxide which is resistant to oxidation. This protective coating isespecially required where the electro magnetic winding will be subjectedto oxidative temperatures or conditions. For example, where the magnetwire is made from aluminum, mere exposure of the wire to air forms athin film of aluminum oxide over the surface of the conductor andprotects it from further oxidation. Where copper magnet wire is used, athin jacket of nickel or aluminum may be disposed over the surface ofthe conductor to prevent its oxidation at high temperatures; thisoxidation-resistant jacket may be either electrodeposited or extrudedonto the surface of the copper magnet wire.

The magnet wire is covered with a flexible abrasionresistant insulationof a fibrous inorganic dielectric material bonded in place by an organicvarnish or enamel alone, or pigmented with one or more finely-dividedglasses. Various fibrous inorganic dielectrics have been used, includingfibers of mica, quartz, and asbestos, but one satisfactory dielectrichas been textile glass fiber manufactured from an alkali-free glass.These textile fibers are soft and lustrous, the individual fibers beingvery small in diameter, and may be bonded to the metallic conductor(especially to copper) with a high-grade insulating varnish to produce asmooth, abrasion-resistant insulation. Particularly satisfactory resultshave been achieved with borosilicate type glasses, both as a pigment inthe varnish and as a serving of fiber glass.

To apply the flexible abrasion-resistant insulation to the metallicconductor prior to forming the winding, the fibrous inorganic dielectricmaterial is served over the wire and the serving coated or impregnatedwith an organic varnish alone, or pigmented with finely-divided glassesto bond it in place. Alternatively, the bare metallic conductor may becoated with varnish and a serving of the fibrous inorganic dielectricimmediately applied to the wet conductor; as the varnish dries, thefiber is bonded to the conductor. In a further alternative, as mentionedabove, an organic varnish impregnated with finely-divided glasses of aparticular particle size may be applied to coat the conductor. Anyalternative may be used to apply the abrasion-resistant insulation tothe conductor since all result in excellent adherence of the inorganicmaterial to the conductor and of the individual fibers to each other.

Selection of a suitable organic varnish or enamel to bond the fibrousinorganic dielectric to the magnet wire may be made from any varnishwhich is capable of bonding the inorganic fiber to the metallicconductor and of bonding the individual fibers to each other and ofholding the finely-divided glasses in suspension, since the primaryfunction of the varnish is to protect the inorganic fiber from crackingor breaking while the coated conductor is being formed into a winding.In addition, the pyrolyd sis point of the particular varnish used (thepyrolysis point being that temperature at which all of the organicmatter in the varnish becomes fugitive or is pyrolytically volatilized),must be appreciably below the fusion point or temperature of theparticular organic fiber used to form the abrasion-resistant insulationabout the magnet Wire. Particularly satisfactory results have beenobtained by using varnishes prepared from polyethylene terephthalate orother polyesters, from cellulose acetate or butyrate or from othercellulosic material, from epoxy resins and from phenolic resins, all ofwhich are completely fugitive at their pyrolysis points.

Partially fugitive organic varnishes, which contain an inorganic moiety,may also be used to bond the fibrous inorganic dielectric to theconductor and thereby form a flexible, abrasion-resistant insulationabout the magnet wire. Upon heating these varnishes to their pyrolysispoint, only the organic matter volatilizes, leaving an inorganic residuewhich forms a matrix with the inorganic fiber. In particular, partiallyfugitive organic varnishes which contain silicon, such as those preparedfrom polysi'loxanes, polysilanes, or polysilcarbanes, have been foundsatisfactory. 0ther varnishes, prepared from organogermanium,organotitanium, or organozirconium polymers, or from coordinationpolymers containing these materials, may be used to bond the inorganicfiber in place to form a flexible, abrasion-resistant insulation in anyof the ways discussed. Since the heat of decomposition of carbon-carbonbonds is generally much lower than the heat of fusion of the lowestmelting inorganic dielectric, all of these varnishes, including thosewhich are completely or even partially fugitive at their pyrolysispoints, may be pyrolytically volatilized from the abrasionresistantinsulation without reaching the fusion point of the inorganic fiber.

After the inorganic insulation has been rendered flexible andabrasion-resistant as described, the magnet wire thus insulated may beformed into an electromagnetic winding of any desired shape withoutdanger of cracking or mechanically destroying the inorganic fiber sincethis fiber is firmly bonded to the underlying conductor. The winding isthen heated to a temperature which is equal to or above the pyrolysispoint of the organic varnish but below the fusion temperature of thefinelydivided glass or the inorganic fiber, the heating being conductedfor a sufficient period of time, preferably under vacuum, topyrolytically volatilize all of the organic matter from the covering,yet leave the formed Winding insulated solely by the inorganicdielectric material. When finely-divided glass is incorporated in theinsulation as described above, the winding is then heated again to ahigher temperature to fuse the finely-divided glass to the conductor orto the inorganic fiber. If, as in the case of glass fiber insulation,this prolonged heating results in the appearance of small cracks orfissures in the final insulation, the quality of the electromagneticwinding is not impaired, since these cracks or fissures only becomeserious when the winding is operated in the presence of excessivemoisture. Inasmuch as very little ambient humidity exists attemperatures of 300 C. or higher, the presence of these small cracks orfissures in the inorganic insulation does not impair the operatingcharacteristics of the electromagnetic winding.

To minimize or even eliminate the formation of these small cracks andfissures which may appear in the final insulation of the electromagneticwinding when glass fiber is used as the inorganic dielectric material,the magnet wire is served over with two different types of glass fibersbonded in place by an organic varnish to form a flexible,abrasion-resistant insulation. One type of glass fiber in thisinsulation has a high fusion point (such as an aluminum borosilicateglass of the type to be discussed below) While the other type possessesa much lower fusion temperature (such as a barium borosilicate glassalso to be discussed below). After the thus-insulated magnet wire hasbeen formed into a winding of any desired shape, the winding is heatedto a temperature which is above both the pyrolysis point of the organicvarnish and the fusion point of the low-fusion type of glass fiber butbelow the fusion temperature of the high-fusion type of glass fiber.This heating is carried out for a sufficient period of time topyrolytically volatilize all organic matter from the covering and topartially fuse the low-fusion glass which, in turn, forms aninterstitial bond between the remaining unfused (high-fusion type) glassfiber, thereby leaving the formed winding insulated solely by glassfiber bonded in place by fused glass.

This same effect can also be achieved by serving the conductor with ahigh-fusion type glass and binding the servings with a slurry containingorganic varnish pigmented with a finely-divided high fusion type glassand a finely-divided low fusion type glass. After forming the winding itis first heated at a temperature above the pyrolysis point of theorganic varnish but below the fusion point of the finely-divided glassesto pyrolitically volatilize all the organic varnish. The winding is thenheated at a temperature above the fusion point of the low fusion typeglass but below the fusion point of the high type glass to fuse thefinely-divided glasses together about the servings thereby leaving theformed winding insulated solely by glass fiber bonded in place by fusedglass.

In some instances it is particularly advantageous to use two diiferenttypes of glass fibers bonded in place by a polysiloxane varnish to formthe flexible, abrasionresistant insulation about the magnet wire. When awinding formed from this magnet wire is heated to a temperature which isabove both the pyrolysis point of the polysiloxane varnish and thefusion point of the low-fusion type of glass fiber, yet is below thefusion point of the high-fusion type of glass fiber, all of the organicmatter from the covering pyrolytically volatilizes while the lowfusiontype of glass fiber partially fuses. The residual silicon dioxideremaining from the pyrolyzed varnish together with the partially fusedglass form a bonding between the remaining unfused glass fibers, therebyleaving the formed winding insulated solely by glass fibers bonded inplace by a matrix of fused glass and residual silicon dioxide.Electromagnetic windings manufactured by this modification of theinvention are capable of being continuously operated at temperatures ofabout 500 C. for extended periods of time.

The temperature to which the winding is heated is, of course, dependentupon the pyrolysis point of the organic varnish which forms part of theflexible, abrasionresistant insulation surrounding the conductor, theonly limitation being that this temperature be below the fusion point ofthe inorganic dielectric material or below the highest fusiontemperature of the inorganic dielectric if more than one is used. In noevent, therefore, should the conductor be insulated with a fibrousinorganic dielectric having a lower fusion temperature than thepyrolysis point of the organic varnish. In general, the winding shouldbe heated to a temperature which is equal to or just above the pyrolysispoint of the organic varnish since this temperature will not usuallyaffect the inorganic dielectric material. In some instances, however,the inorganic dielectric material may suffer if it is subjected totemperatures which are far above the intended operating temperature ofthe winding, and consequently sufiicient control should be exercisedduring the heating of the Winding to prevent any change in thedielectric properties of the inorganic dielectric material.

Preferred embodiments of the invention are described hereinbelow withreference to the drawings wherein:

FIG. 1 is an enlarged section of a copper wire sheathed with a thinlayer of nickel and coated with unfired enamel silicone and powderedglass;

FIG. 2 is an enlarged section showing the copper wire of FIG. 1 after ithas been fired leaving the wire coated with fused glass;

FIG. 3 is an enlarged section showing copper wire in the unfired statewhich is coated with a nickel coating and has three coatings of siliconeand powdered glass which are separated by glass fiber servings; and

FIG. 4 is an enlarged section of the copper wire of FIG. 3 in the firedstate showing the fibers and fused glass coating the wire.

In one specific embodiment of the invention, an electromagnetic windingwas manufactured from a bare copper magnet wire which had previouslybeen sheathed with a thin but continuous layer of nickel to preventoxidation of the conductor at the high operating temperatures to whichthe winding was to be subsequently exposed. This magnet wire was servedover with braided fiberglass and then immersed in a solution ofpolyethylene terephthalate dissolved in a naphtha-cresol solvent pair.After wiping excess solution from the coated wire, it was passed througha vacuum drying oven to remove all traces of solvent and bond thefiberglass to the conductor and thereby produce a smooth,abrasion-resistant surface. The magnet wire was then introduced into awinding apparatus where, under slight tension, it was formed into awinding of the desired shape, during which time the individual glassfibers contained in the insulation neither separated from each other norfrom the conductor, even though they were subjected to considerablebending, friction and wear. The winding (and a sufficient number ofothers similar to it) was inserted into its proper slot in an electricgenerator, and the entire machine was then placed in a vacuum oven andheated to a temperature of about 500 C. until all of the organic matterhad been pyrolytically volatilized and removed from the coating coveringthe winding. After all of the organic matter had been removed, thegenerator was cooled very slowly to prevent the formation of fissuresand cracks in the glass insulation and to anneal the nickel-platedmagnet wire which had become slightly hardened during the windingoperation. This machine could be continuously operated at temperaturesfrom 300 C. to 500 C. without any danger of thermally-induced failure.

In another specific embodiment of the invention a wet enamel or slurrywas applied to an oxidation resistant bimetallic conductor (eg a copperconductor clad with nickel, aluminum or stainless steel) in theconventional manner with a ball or grooved die. Multiple coats wereapplied interspersed with oven baking steps in a continuous operation.

The slurry consists of a liquid polyester modified diphenyl siloxane(silicon resin) pigmented in a 4:1 resin to pigment ratio by weight,with finely-divided inorganic material. This finely-divided inorganicmaterial is preferably formed from a low melting borosilicate type glassand is preferably in either a flake, powder, or finelydivided fiberform. It has particularly been found that the formation of glass fibersin a limited range of particle sizes has contributed appreciably to thecommercial production of this fine Wire.

In one example a barium borosilicate glass in the following composition:

SiO 27.7 CaF 6.4 ZnO 5.3 CaO 4.5 BaO 17.2 Na O 11.7 B 0 26.3

was obtained in a felt-like sheet formed of fibers which are initially 1micron or less in diameter and approximately 1 mil long. The felt is cutinto short strips and admitted to a pebble mill with xylol. The glass ismilled in the ceramic barrel of the pebble mill for approximately 3-4days until the average length of the glass fibers is 25 microns. Thisgrinding has not been found to affect the cylindrical or fiber nature ofthe glass; it seems only to shorten the fiber lengths.

It has been found that glass fibers in a range of from 0.05 to 1.0micron in diameter and 1-10 microns in length depending on the diameter(the ratio of length to diameter being approximately :1) allows theformation of a mat structure (similar to paper) in spite of the reducedlength of the fibers. This mat structure effect is found to beespecially desirable in that it lessens the tendency of thefinely-divided glass to fall off the wire in the pyrolysis operationwhere the resin burned off. This falling off of the finely-divided glasshas been particularly troublesome when the glass is in flake or powderform.

After the conductors have been coated with the desired amount of slurrythe wires are formed into a winding and are submitted to a pyrolysisstep wherein they are heated at 500 C. for approximately A hr. Thispyrolysis step burns off the resin and causes the silicon resins toleave an ash which is rich in silicon dioxide but does not fuse the lowmelting glass fibers. This silicon dioxide aids in promoting theadhesion necessary to hold the glass to the conductor surface before andafter the final sintering step. This final sintering step involvesfiring the coated conductor at 650 for /2 hr. whereby the glassparticles are fused and form a mass of glass insulation between theinterstices of the adjacent conductors.

In an alternative embodiment an oxidation resistant bimetallic conductorwas coated with a base coat of enamel. This enamel can be of the typepigmented with finely divided inorganic material of the type used on theconductor as described above. The base coat is then covered with atleast one and preferably several servings of continuous-filament fiberglass. A slurry consisting of the same liquid resin used for the enamelwire plus a twocomponent glass pigment system in a finely-divided stateis then applied over the servings of fiber glass to bind them in placeand thereby form a flexible abrasion-resistant insulation about theconductors. It has been found that the glass fiber servings arepreferably composed of aluminum borosilicate glass of the followingcomposition:

SiO 53.9 A1203 14.2 Na O' 0.36 K20 04 01,0 0.1 CaO 21.7 B203 8.69 F5203so, 0.05 F2 0.44

One component of the two-component glass pigments in the finely-dividedstate is also an aluminum borosilicate type glass which is in flakeform. The second component is a barium borosilicate glass of the typementioned above in regard to the enamel Wires. The barium borosilicatetype glass has a lower melting point than the aluminum borosilicateglass does. In one example the slurry used to bond the fiber glassservings had the following composition:

Aluminum borosilicate glass flake 27.8 Barium borosilicate glass powder18.6 Silicone (50% solids) 42.0 Xylol 5.8 Cresylic acid 2.4 Lithiumnitrate 3.4

After the servings are bonded with the slurry, the wires are formed intoa winding and subjected to a pyrolysis step wherein the volatile resinis burned off. This pyrolysis step is carried out at a temperature belowthe fusion point of the low-fusion type glass. The winding is thensubjected to a sintering step wherein it is heated at a temperatureabove the fusion point of the barium borosilicate glass but below thefusion point of the aluminum borosilicate glass. As a result, during thesintering step the barium borosilicate glass component melts and fusesthe flakes and fibers into a mass thereby leaving the formed windinginsulated solely by inorganic dielectric material.

In any of these embodiments a phase insulation may be incorporated intothe windings. One type phase insula-- tion used consists of glass fibercloth or paper treated on both sides and thoroughly impregnated with aslurry of a silicone varnish pigmented with a fusible glass powder ofthe types mentioned. The slurry used in this case can be pigmented witheither the two component borosilicate type glasses used to bond thefiber glass servings, or, about. 46% iron glass could be substituted forthe borosilicate glasses. It is also intended that mica paper might alsobe used. Upon heating, the silicone varnish will pyrolyticallyvolatilize and further heating will cause the fusible glass to fuseabout the cloth or paper, thereby forming efiective phase insulation.

It is also contemplated that a potting compound con-- sisting of a lightsilicone varnish impregnated with a powdered glass fusible at a lowertemperature than those mentioned above might also be applied around thecoil after it has been formed and heated. After coating the coil withthis compound the coil would again be subjected to a heating operationin which the organic varnish would volatilize and the finely-dividedglass would fuse, thereby providing a very effective moisture seal.

It is further intended that additives could be added to the organicvarnish or could be separately applied to the conductor and then coatedwith an organic varnish pigmented with the finely-divided glass. Amongthe additives suggested are alumina, quartz, silicates of aluminum,calcium, magnesium, sodium aluminum, lithium aluminum, zirconium, mica,magnesia, titania, talc, and flint. Thus, it is intended that any of theslurries mentioned above could have additives such as these Withoutdeparting from the scope of the invention.

We claim:

1. An electromagnetic winding capable of operation at temperatures above300 C. comprising a bimetallic copper conductor with nickel or stainlesssteel cladding wound into a coil and having, as its insulation, a fusedmat of borosilicate glass.

2. An electromagnetic winding capable of operation at emperatures above300 C. comprising a conductor wound into a coil and having, as its soleinsulation, a serving of glass fibers and residual silicon dioxideremaining from the pyrolytic volatilization of a polysiloxane varnish,the residual silicon dioxide forming a matrix throughout the intersticesof the glass fibers.

3. An electromagnetic winding cap-able of operation at temperaturesabove 300 C. comprising a copper conductor with nickel or stainlesssteel cladding wound into a coil and having, as its sole insulation, aserving of glass fibers bonded in place by a matrix of fused glass andsilicon dioxide, said silicon dioxide being the residue of pyrolyticvolatilization of a polysiloxane varnish, and the fusion point of thefused glass being lowerthan that of the glass fibers.

, 4. An electromagnetic winding capable of operation at temperaturesabove 300 C. comprising a copper conductor with nickel or stainlesssteel cladding wound into a coil and having, as its sole insulation, aserving of glass fibers bonded in place by fused glass having a fusionpoint lower than that of the glass fibers.

5. An electromagnetic winding capable of operation at temperatures above300 C. comprising a bimetallic copper conductor with nickel or stainlesssteel cladding wound into a coil and having, as its insulation, aserving of fibrous inorganic fibers bonded in place by a firstborosilicate glass and fused to a second borosilicate glass having amelting point lower than said first borosilicate glass.

6. An electromagnetic winding capable of operation at temperature above300 C. comprising a bimetallic copper conductor with nickel or stainlesssteel cladding Wound into a coil and having, as its insulation, aserving of fibrous aluminum borosilicate glass bonded in place by a massof aluminum borosilicate glass and fused to a barium borosilicate glass,said beryllium borosilicate glass having a fusion point lower than saidaluminum borosilicate glass.

References Cited UNITED STATES PATENTS Wil'kotl 174110 X Steinbock 10650Whearley 174121 Whearley 174--122.1 Grimshaw 174-420 LEWIS H. MYERS,Primary Examiner.

E. GOLDBERG, Assistant Examiner.

1. AN ELECTROMAGNETIC WINDING CAPABLE OF OPERATION AT TEMPERATURES ABOUT300*C. COMPRISING A BIMETALLIC COPPER CONDUCTOR WITH NICKEL OR STAINLESSSTEELL CLADDING WOUND INTO A COIL AND HAVING, AS ITS INSULATION, A FUSEDMAT OF BOROSILICATE GLASS.